We built the subfloor in our camper van conversion for stability and longevity to “stand” the test of time. Down the road, if we want to change our layout and refresh the van, we want to just re-arrange some cabinets. We don’t want to make any changes to the subfloor!
Insulating the floor is important – but it isn’t as important as insulating our walls & ceiling. The warmth inside our van will be rising. Every inch of insulation we add is an inch of headroom we lose in the van. We chose 2” of insulation in the ceiling (our Ceiling Panel Installation) and 1” in the flooring.
We used polyiso insulation for two reasons. First, it has the highest R Value per inch (6.0) of any rigid insulation. Second, it has high compressive strength of 16-25 pounds per square inch. This means we and all our friends can (and will!) stand and jump on our floor without fear of it collapsing.
Attaching Joists to Van
Some builders screw or bolt their joists into the van metal. We did not consider it necessary. Sikaflex 221 adhesive on the joists secures the subfloor to the van.
Cabinets and interior elements are bolted to van walls, not the floor. In a car accident, these hold our cabinets and everything in place. The subfloor is for standing on and that’s it. It doesn’t provide structural integrity for any of our build
High strength glue will last many years. Gravity is on its side! If glue fails, joists still won’t move as they are each screwed wide spans of plywood.
Screwing through the van metal floor was undesirable as it would create potential rust/leak spots
A van build calls for lots of adhesive, and the best choice is Sikaflex-221. Its insanely strong on metal, water resistant, non-corrosive, lower-VOC, and flexible (it’s in the name).
Awkward Van Spot
The Transit’s van metal floor has ribs. These are almost 0.5” tall, but not quite! We tried to fill those low spots with something 0.5” tall. But the ribs are shorter than 0.5″, so that filler did not create a level surface.
In the end, we placed 1.5” tall lumberin the low spots as joists, and 1” polyiso sitting on the ribs. The lumber was 2” x 3” (actual measurement 1.5” x 2.5”).
Thickness of Plywood for Van Sub-Floor
In standard construction, subfloors are ¾ inch plywood or thicker. But we want as thin as reasonable. Benefits of our 1/2″ plywood include lighter weight, lower cost, and an extra ¼ inch of headroom!
The spacing of the joists supporting the floor determines the plywood thickness.
How far apart should the joists be, both side-to-side and front-to-back? We can say that our subfloor feels super sturdy! Side-to-side spacing between our joists is 18 inches on center at its greatest. This span rating for plywood defines how far apart joists can be. The span rating for 1/2 inch plywood is 32/16. That’s 32” when plywood is sheathing for a roof (not much expected weight on it), and 16 inches as a floor (lots of expected weight on it).
We’re comfortable slightly exceeding the recommended subfloor spacing. The interior of our van is (hopefully) quite light. Also, the polyiso is sitting about 1/8″ underneath our joists. The polyiso’s high compressive strength means it could support any plywood sag.
Span ratings for standard sheathing plywood are printed in big black letters. These are good to know, but we don’t intend to subject our van to the same stresses a commercial building undergoes. The following numbers represent “roof sheathing span / subfloor span” per the APA – The Engineered Wood Association (exciting group!)
Layout of Floor Joists
We are ever mindful of weight and cost, which is why we took the time to cut our joists into smaller pieces. We could have used long 12’ foot lumber from front to back, but that’s not necessary!
The edges of the plywood sheets are their weakest spot. We used four 4×8 sheets of plywood.
A joist is necessary underneath all joints/edges of plywood. First we measured and planned where the edges would land. We ensured a joist would bridge the gap between the plywood sheets.
Our main entryway at the van sliding door will endure a lot of traffic and it needs support. However there is a metal rib in the way of putting a normal joist there. Thus we notched the underside 0.5” depth with a table saw. We used a 2”x4” for this so that it spanned the width of the rib. Now the 2”x4” is sitting on the van metal floor. Later we used 1″ screws to attach the plywood subfloor to this joist due to its shorter depth after the notch. We used 1.5″ screws elsewhere.
Awkward Van Spot
The back one foot or so of the van metal floor is not flat and curves up (and down) ever so slightly. This was not significant enough for any drastic alterations, and our solution was:
Make the back joists very short (4” or so). Thus the unlevel metal was in contact with the shortest possible material
Glob lots of adhesive on the portions of the joists that wouldn’t be flush with metal. This hopefully fills in any gap between the wood and metal
Use a plywood that was slightly bowed. The goal is that the plywood’s bow matches the upward curve of the joists.
Installing Joists on the Van Floor
Simple – we put adhesive on the lumber, set them on the metal, and put some weight on top of them! But we committed a rookie mistake. We used standard Liquid Nails adhesive because we had some on hand.
We need to read the label first, and make sure the adhesive is compatible with metal. Metal needs a stronger adhesive than other materials since it is not porous. We did it right the second time and used Sikaflex.
Installing Rigid Insulation
For maximum coverage and strength, we used full 4’x8’ sheets of polyiso insulation. We notched spaces for the joists in the insulation. There’s a simple trick to this!
Cut polyiso to size, including wheel well area and notches for various vertical ribs
Paint the joists
Place the polyiso on top of the wet joists
Voila, flip your polyiso over and cut out the parts where the wet paint marked the insulation!
We ran our polyiso perpendicular to the plywood (polyiso is front to back, plywood is side to side). This maximizes stability in the final floor.
The easiest way to cut polyiso insulation is to heat up a butter knife until red hot (we used a propane torch). The knife will slice like butter. A saw will tear the insulation which is much less pleasant or precise.
Safety Note. Cutting polyiso with a hot knife produces vapor/fumes that smell funny. We assume they are unhealthy to inhale. In our picture Nadia is wearing a dust mask. In reality this does very little to stop these fumes because the dust mask is too porous.
We placed aluminum foil tape over all the polyiso joints to complete the vapor barrier in the floor. We ended up abandoning the plan for a vapor barrier for the rest of the van. In the end we don’t believe this step is necessary.
Awkward Van Spot
The spare tire is under the van, and is lowered by turning a bolt housed in a compartment with a black cover. The lug wrench and jack handle are in compartment below passenger seat. We wanted to bring our flooring all the way to the back of the van. So, we need to keep this compartment on the metal floor accessible!
Using a 2″ holesaw, we made a hole in our subfloor and final floor to keep this accessible.
Cutting and Installing Plywood Subfloor
We cut the 4×8 sheets of ½ inch plywood to size using a circular saw. We left approximately 1/16″ inch of space between each plywood sheet, and between the plywood and the van walls. These expansion gaps are necessary as the plywood will naturally expand and contract. If there is no gap, then the plywood will bend and buckle when it expands.
Next came templates for the wheel wells, and we measured the notches for any other necessary cut outs. Fortunately these do not need to be perfect. 1) expanding foam will fill the gaps, 2) a flooring material will be on top of all the plywood anyways, and 3) wheel wells won’t be visible from the living area!
Now comes the fun part, fastening the plywood! We used 1.5” screws that countersink (any screw that has a cone head instead of a flat head). We started with the front plywood panel. We measured the joists’ locations in relation to the the front and the walls. These measurements were transferred to the plywood. These marks indicated where screws should be placed. For the other three plywood panels, the process was easier:
Completing the Subfloor: Trim and Spray Foam on Edges!
We sprayed expanding spray foam along the edges of the plywood where they meet the van walls. These gaps were anywhere from 1/8″ – 1/2″. Once dried, the spray foam was carved down to be flush with the plywood.
We finished the edges with aluminum angle. The aluminum was screwed down through the floor into joists. We added the trim after installing our cabinets and vinyl plank flooring (in that order). Yep, we had to ensure there were joists installed on the edges of the floor. This ensured stability at vulnerable weak points at the edges, and material for screwing in this aluminum angle.
When you’re done with your comfortable subfloor, enjoy macarons and sparkling white grape juice with friends!
We sincerely hope this information is helpful on your build journey!
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